1. Field of the Invention
This invention relates to a method and apparatus for manufacturing a disc-shaped recording medium, such as a magneto-optical disc. More particularly, it relates to a method and apparatus for applying a coating liquid, such as a material for a protective film, on a disc-shaped recording medium.
2. Description of Related Art
The photomagnetic recording system is such a system in which a recording layer of a magnetic material is locally heated on light irradiation to a temperature higher than the Curie temperature or temperature compensation point to reduce the coercivity in this portion and in which a recording magnetic field is applied to this portion from outside to change partially the direction of magnetization of the recording layer to record information signals. This photomagnetic recording system is finding practical use in an optical filing system, an external storage device for a computer or in a recording device for the audio and video information.
Among recording mediums recorded or reproduced by the above photomagnetic recording system, there is a magneto-optical disc comprised of a recording layer formed on a transparent substrate formed of plastics, such as polycarbonate, plastics or glass. The recording layer of a magneto-optical disc is usually comprised of a magnetic layer for recording information signals thereon and a dielectric film, layered on the magnetic layer, and a light reflective layer of, for example, aluminum, formed as an uppermost layer. The magnetic layer of the magneto-optical disc used has an easy axis in a direction perpendicular to the film surface and exhibits a significant photomagnetic effect. Among these magnetic films, there are magnetic thin films comprised of, for example, a amorphous thin film of an alloy of a rare earth-transition metal. On the light reflecting film laminated on the upper surface of the recording layer is usually formed a protective film of, for example, a UV curable resin, for preventing corrosion or damage in the recording layer.
The magneto-optical disc is not limited to a single-plate type but may also be a double-plate type magneto-optical disc obtained on bonding two magneto-optical discs with their recording layer sides or substrate sides facing each other. With the double-plate type magneto-optical disc, since signals are recorded independently on the recording layers of the respective discs, the recording capacity is double that of the single-plate type magneto-optical disc. Also, since the double-plate type magneto-optical disc is symmetrical with respect to its bonding surface, it is less susceptible to warping or deformation of the substrate against changes in temperature or humidity of the external environment than the single-plate type magneto-optical disc.
The photomagnetic recording system may be roughly classified into a light modulating type of modulating the light for recording signals and a magnetic field modulating type of modulating a recording magnetic field for recording signals. Of these, the magnetic field modulating type system is such a system in which, when recording signals, the recording magnetic field is modulated at an elevated speed for recording signals on the recording layer as the light is continuously illuminated on the disc. Researches are conducted energetically in the magnetic field modulating system since it permits facilitated overwriting, high density recording and fast accessing.
In the magnetic field modulation system, a magnetic field is applied against the recording layer during signal recording by a magnetic head adapted for generating a recording signal magnetic field. Since the magnetic field needs to be inverted at an elevated speed during signal recording, it is not possible to generate an excessively strong magnetic field across the magnetic head used for recording in the magnetic field modulating system. The strength of the magnetic field applied by the magnetic head on the magnetic head is inversely proportionate to the distance between the magneto-optical disc and the magnetic head. That is, the strength of the magnetic field applied across the magnetic head to the magneto-optical disc becomes smaller the larger the distance between the magneto-optical disc and the magnetic head. Thus, in this magnetic head, it is necessary to reduce the distance between the head and the magneto-optical disc for signal recording. Therefore, when the magnetic field is applied across the magneto-optical disc by the magnetic head, it is in general more desirable to apply the magnetic field from the side of the protective film thinner in film thickness than the substrate, than from the side of the substrate.
Also, when illuminating the laser light on the recording layer of the magneto-optical disc from an optical head, a thin film thickness of a light transmitting layer from the light incident surface to the recording layer of the magneto-optical disc is desirable for suppressing occurrence of aberration. Thus, it is more preferred to arrange the optical head on the side of the protective film thinner in film thickness than the substrate to illuminate the laser light from the side of the protective film for laser light illumination, than arranging the optical head on the substrata side.
That is, both the magnetic head and the optical head are preferably arranged on the protective film side of the magneto-optical disc. Thus, there is contemplated an information recording/reproducing system in which an optical head for illuminating a laser light beam on the magneto-optical disc and a magnetic head for applying a magnetic field across the magneto-optical disc are unified and both the optical head and the magnetic head are provided on the protective film side of the magneto-optical disc.
If the optical head and the magnetic head are unified and provided on the protective film side of the magneto-optical disc, the laser light beam from the optical head is illuminated on the recording layer without interposition of the substrate. Thus, the material of the magneto-optical disc is not limited to a transparent material, that is, a substrate formed of an opaque material may be used. Therefore, such a material that is less susceptible to warping or deformation, such as Al, can be used as a substrate. Specifically, if the optical head and the magnetic head are unified and arranged on the protective film side of the disc, it is possible to use a material less susceptible to warping or deformation, such as Al, as a substrate material to prevent warping of the substrate from occurring.
On the light reflecting surface, deposited as an uppermost recording layer, there is formed a protective layer of, for example, UV curable resin, for the purpose of preventing corrosion or damage to the recording layer. This protective film is usually formed by applying a UV curable resin by so-called spin coating.
For forming a protective film by the spin coating method, a disc substrate 101 having a recording layer formed thereon is set on a turntable 102 which is rotated by a spindle motor at a low speed, as shown in FIG. 1. A UV curable resin 103 is applied in a toroidal fashion along the inner rim of the recording area of the disc substrate. The disc substrate 10 is then run in rotation at an elevated speed to stretch the UV curable resin up to the outer rim to form a coating layer of the UV curable resin on the entire surface of the disc substrate 101. The coating film of the UV curable resin is then cured on illumination of UV light to form a protective film on the disc substrate 101. The thickness of the protective film is of the order of approximately 15 .mu.m for achieving a sufficient protective effect.
If the protective film is formed as shown in FIG. 1, the protective film tends to be thicker in film thickness in a direction towards the outer rim of the disc substrate 1. If the coating film is formed by the spin coating method, the comprehensive film thickness of the coating film can be changed by changing operating conditions, such as viscosity of the coating liquid, herein the UV curable resin, the rpm of an article for coating, herein the disc substrate, or the rotating time duration. However, if the protective film is formed as shown in FIG. 1, it is difficult to assure a uniform thickness of the protective film over the entire surface of the disc substrate, even if the operating conditions during application of the protective coating material are adjusted, such that the film thickness on the outer rim of the disc substrate 1 is thicker than that on its inner rim.
FIG. 2 shows the film thickness distribution when the protective film is formed with the viscosity values of the UV curable resin of 500, 140 and 37 cps. In FIG. 2, the ordinate and the abscissa denote the film thickness of the protective film and the radial position from the center of the disc substrate, respectively. In FIG. 2, curves A, B anc C denote measured values of the film thickness of the protective film and the radial position from the center of the disc substrate, when the protective film is formed by a UV curable rein with the viscosity of 500 cps, 140 cps and 37 cps, respectively.
The relation between the film thickness of the protective film and the radial position from the center of the disc substrate, shown in FIG. 1, was measured on a protective film produced by supplying the UV curable resin at a radial position of 17 mm from the center of the disc substrate in a toroidal fashion, increasing the rpm to 3000 over a second, keeping the disc substrate rotating at 3000 rpm for 8 sec and subsequently illuminating the UV rays thereon.
The film thickness of the protective film produced on applying the UV curable resins of different viscosities under the same condition is known to be proportionate to the square root of the viscosity. Thus, the film thickness of the protective film produced from the UV curable resin with the viscosity of 140 cps and that of the protective film produced from the UV curable resin with the viscosity of 500 cps under the same conditions theoretically bear a ratio of (500/140).sup.1/2 =1.9. Similarly, the film thickness of the protective film produced from the UV curable resin with the viscosity of 37 cps and that of the protective film produced from the UV curable resin with the viscosity of 500 cps under the same conditions theoretically bear a ratio of (500/37).sup.1/2 =3.7.
FIG. 3 shows a first normalized film thickness distribution, calculated by multiplying the film thickness of the protective film formed of the UV curable resin with the viscosity of 140 cps by 1.9, and a second normalized film thickness distribution, calculated by multiplying the film thickness of the protective film formed of the UV curable resin with the viscosity of 37 cps by 3.7, based on the above-mentioned film thickness ratio. In FIG. 3, the film thickness distribution of the protective film produced from the UV curable resin with the viscosity of 500 cps is indicated by .smallcircle., while the first normalized film thickness distribution and the second normalized film thickness distribution are denoted by .DELTA. and .quadrature., respectively.
Referring to FIG. 3, the first normalized film thickness distribution, second normalized film thickness distribution and the film thickness distribution of the protective film formed from the UV curable resin with the viscosity of 500 cps are substantially on the same curve. Therefore, it is seen that the film thickness of the protective film is proportionate to the square root of the viscosity, other conditions being the same.
It is also seen from FIG. 3 that if, with the spin coating method routinely used as a protective film forming method, a protective film is to be formed to a predetermined film thickness, there is produced a film thickness difference of a pre-set value between the film thickness on the inner rim and that on the outer rim of the disc, even if the operating conditions, such as rpm or the rotating time duration, are changed.
It should be noted that the film thickness difference of a pre-set value between the film thickness on the inner rim and that on the outer rim of the disc can be decreased by employing a UV curable resin of low viscosity, lengthening the rotation time duration of the disc substrate and by reducing the film thickness of the applied UV curable resin over the entire surface o the disc substrate. For example, the film thickness difference between the film thickness at a radial position of 24 mm and that at a radial position of 40 mm from the center of the magneto-optical disc can be reduced to approximately the order of 1.5 .mu.m by employing the UV curable resin of 37 cps, lengthening the rotation time duration and by reducing the film thickness of the protective film in its entirety.
However, if the film thickness of the protective film is reduced excessively, it becomes impossible to prevent corrosion of the recording layer of the magneto-optical disc. Therefore, the film thickness of the order of 15 .mu.m on an average at the minimum is required on the entire surface of the protective film. If the average film thickness of the protective film over the entire surface of the magneto-optical disc is 15 .mu.m, a film thickness difference of approximately 5 .mu.m is produced between the inner and outer rims even if the UV curable resin of low viscosity is used and the rotation time duration of the disc substrate is lengthened.
If the film thickness difference is produced between the film thickness on the inner rim and that on the outer rim of the magneto-optical disc in this manner, there is produced wavefront aberration in the laser light illuminated on the recording layer if an optical head in which the laser light is illuminated from the protective film forming surface for recording/reproduction is used. If the wavefront aberration is produced in an optical head due to film thickness difference of the protective film, the laser light illuminated on the recording layer is unstable to deteriorate the recording/reproducing characteristics.
If the refractive index of the protective film is n, the film thickness error of the protective film is .DELTA.d and the numerical aperture of the objective lens used for converging the laser light on the recording layer is NA, the wavefront aberration W.sub.4od is given by the following equation (1): ##EQU1##
With the current magneto-optical disc, the refractive index n of the protective film is 1.58, the laser wavelength .lambda. of the optical system is 780 nm and the numerical aperture of an objective lens is 0.5. If, under this condition, the film thickness error .DELTA.d of the protective film is 5 .mu.m, the wavefront aberration W.sub.4od is 0.19.lambda. (=0.148 .mu.m), as may be found from the equation 1.
Meanwhile, with the increased recording density in recent years, it is practiced in an optical system for recording/reproducing a magneto-optical disc to shorten the laser wavelength .lambda. of the optical system and to increase the numerical aperture of the objective lens. This is because the spot diameter of the laser light converged on the recording layer is proportionate to the wavelength .lambda. of the laser light and inversely proportionate to the numerical aperture NA of the objective lens.
For example, in the current magneto-optical disc, the laser spot diameter is approximately of the order of 1.6 .mu.m. If, for example, the laser light wavelength is 480 nm and the numerical aperture NA of the objective lens is 0.9, the laser spot diameter is 0.6 .mu.m, so that the spot diameter is approximately one-third of the current laser spot diameter. Thus, if this optical system is used, the surface recording density of the magneto-optical disc can be increased to approximately 9 times that of the disc now in use.
However, if, when there is a film thickness error of the order of approximately 5 .mu.m in the above-described protective film, the optical system of the short wavelength and the high numerical aperture is used, the wavefront aberration W.sub.4od calculated from the equation 1 is of a larger value, such that stable recording/reproduction cannot be realized. For example, if ,with the wavelength of the laser light beam of 480 nm and with the numerical aperture NA of the objective lens of 0.9, the film thickness error of the protective film is as much as 5 .mu.m, the wavefront aberration W.sub.4od is increased significantly, so that stable recording/reproduction cannot be achieved. If, with the optical system of such magneto-optical disc, the wavefront aberration W.sub.4od is suppressed at most to the order of 0.19 .lambda. equal to that of the currently used disc, the film thickness difference .DELTA.d between the inner and outer rims of the disc needs to be not more than 2.9 .mu.m.
Also, the film thickness difference in the protective film described above affects not only illumination of the laser light by the above-described optical system but also the application of the magnetic field by the magnetic head. Among the types of the magnetic head used in the above-described magnetic field modulation system, there are, for example, a floating type magnetic head recording signals in a state slightly floated by approximately tens of namometers to tens of micrometers above the protective film of the magneto-optical disc, and a sliding type magnetic head recording signals as it slides over the protective film of the magneto-optical disc. If, in the magnetic field modulation system employing these magnetic heads, there is a film thickness difference in the protective film, the separation between the recording layer and the magnetic head is changed. That is, if, with the use of the floating type magnetic head or the sliding type magnetic head, there is a film thickness difference in the protective film, the separation between the recording layer and the magnetic head is nonuniform to reflect the film thickness difference. Therefore, with the magneto-optical disc employing the floating type magnetic head or the sliding type magnetic head, in particular, it is strongly desired to suppress fluctuations in the film thickness of the protective film to as small a value as possible.
FIG. 4 shows the relation between the magnetic field applied across the recording layer of the magneto-optical disc and the separation between the magnetic head and the recording layer. As may be seen from FIG. 4, if the separation between the magnetic head and the recording layer is varied, there is produced variation in the strength of the magnetic field applied across the recording layer. For example, if, with the use of the floating type magnetic head having a floating height of 5 .mu.m, there is a film thickness difference of 5 .mu.m between the inner and outer rims of the disc, there results a difference in the strength of the applied magnetic field of the order of 15 Oe between the inner and outer rims of the disc.
As a method for suppressing the variation in film thickness, there is proposed a method of forming a protective film as UV light is illuminated on a magneto-optical disc run continuously in rotation at an rpm not less than 1000 rpm. However, it has not been possible with this method to eliminate the variation in the film thickness of the protective film completely such that there is produced a small film thickness difference between the inner and outer rims of the disc.